1. Field of the Invention
The present invention generally relates to photo-stable highly luminescent chromophores which are useful in various applications, including in wavelength conversion films. Wavelength conversion films have the potential to significantly enhance the solar harvesting efficiency of photovoltaic or solar cell devices.
2. Description of the Related Art
The utilization of solar energy offers a promising alternative energy source to the traditional fossil fuels, and therefore, the development of devices that can convert solar energy into electricity, such as photovoltaic devices (also known as solar cells), has drawn significant attention in recent years. Several different types of mature photovoltaic devices have been developed, including Silicon based device, III-V and II-VI PN junction devices, Copper-Indium-Gallium-Selenium (CIGS) thin film devices, organic sensitizer devices, organic thin film devices, and Cadmium Sulfide/Cadmium Telluride (CdS/CdTe) thin film devices, to name a few. More detail on these devices can be found in the literature, such as Lin et al., “High Photoelectric Conversion Efficiency of Metal Phthalocyanine/Fullerene Heterojunction Photovoltaic Device” (International Journal of Molecular Sciences, vol. 12, pp. 476, 2011). However, the photoelectric conversion efficiency of many of these devices still has room for improvement and development of techniques to improve this efficiency has been an ongoing challenge for many researchers.
One technique developed to improve the efficiency of photovoltaic devices is to utilize a wavelength conversion film. Many of the photovoltaic devices are unable to effectively utilize the entire spectrum of light as the materials on the device absorb certain wavelengths of light (typically the shorter UV wavelengths) instead of allowing the light to pass through to the photoconductive material layer where it is converted into electricity. Application of a wavelength conversion film absorbs the shorter wavelength photons and re-emits them at more favorable longer wavelengths, which can then be absorbed by the photoconductive layer in the device, and converted into electricity.
This phenomenon is often observed in the thin film CdS/CdTe and CIGS solar cells which both use CdS as the window layer. The low cost and high efficiency of these thin film solar cells has drawn significant attention in recent years, with typical commercial cells having photoelectric conversion efficiencies of 10-16%. However, the energy gap of CdS is approximately 2.41 eV, which causes light at wavelengths below 514 nm to be absorbed by CdS instead of passing through to the photoconductive layer where it can be converted into energy. This inability to utilize the entire spectrum of light effectively reduces the overall photoelectric conversion efficiency of the device.
There have been numerous reports disclosing the utilization of wavelength conversion materials to improve the performance of photovoltaic devices. For example, U.S. Patent Application Publication No. 2009/0151785 discloses a silicon based solar cell device which contains a wavelength down-shifting inorganic phosphor material. U.S. Patent Application Publication No. US 2011/0011455 discloses an integrated solar cell comprising a plasmonic layer, a wavelength conversion layer, and a photovoltaic layer. U.S. Pat. No. 7,791,157 discloses a solar cell with a wavelength conversion layer containing a quantum dot compound.
While there have been numerous disclosures of wavelength conversion inorganic mediums used in photovoltaic devices and solar cells, there has been very little work reported on the use of photo-luminescent organic mediums for efficiency improvements in photovoltaic devices. The use of an organic medium, as opposed to an inorganic medium, is attractive in that organic materials are typically cheaper and easier to use, making them a better economical choice. Some theoretical modeling and/or simulation of luminescent films applied to CdS/CdTe solar cells is described in the following literature, U.S. Patent Application Publication No. 2010/0186801, B. S. Richards and K. R. McIntosh in “Overcoming the Poor Short Wavelength Spectral Response of CdS/CdTe Photovoltaic Modules via Luminescence Down-Shifting: Ray-Tracing Simulations” (Progress in Photovoltaics: Research and Applications, vol. 15, pp. 27-34, 2007), and T. Maruyama and R. Kitamura in “Transformations of the wavelength of the light incident upon solar cells” (Solar Energy Materials and Solar Cells, vol. 69, pp. 207, 2001).